a) Field of the Invention
The present invention relates to a dynamic-pressure fluid bearing, such as a pneumatic bearing and liquid bearing, to be installed in a motor to drive a polygon mirror or hard disk.
b) Description of the Related Art
There are special motors, such as those for driving a polygon mirror or hard disk, which need high precision for high speed rotation. This requirement is met by the use of a dynamic-pressure fluid bearing which is lubricated with air or oil. An example of the motor driving a polygon mirror is shown in FIG. 2. A polygon mirror scans the laser beam in a digital copying machine or laser printer. The motor runs at 10000-30000 rpm.
To realize such a high rotary speed, the motor is provided with a dynamic-pressure fluid bearing which supports the rotor without contact. In FIG. 2, there is shown a polygon mirror 1 which is fitted on a projection 13 at the end of a rotor 2. The polygon mirror 1 is pressed against (and hence fixed to) the rotor 2 by a corrugated spring 17 held between it and a balancing plate 16. The rotor 2 is rotatably fitted in the bearing 5. The rotor 2 has spiral grooves 15 formed in its outer surface 14. Between the outer surface 14 of the rotor 2 and the inner surface of the bearing 5 is formed a dynamic-pressure pneumatic bearing which supports the rotor 2 rotating at a high speed. Both the rotor 2 and polygon mirror 1 are made of aluminum alloy, which is easy to machine.
For improvement on wear resistance, the outer surface of the rotor 2 is coated with electroless composite plating of SiC and nickel and the inner surface of the bearing 5 is coated with lubricating anodized film 26. (See Japanese Patent Laid-Open No. 235719/1988 filed by the present applicant.)
In the rotor 2 are an annular magnet 10 (forming the driving magnetic circuit) and an iron yoke 19. The iron yoke 19 is a bottomed cylindrical body which is fixed to the rotor 2 together with the balancing plate 16 by a screw 18 (with its head recessed) driven into a screw hole 20 in the bottom. Fixing by screws may be replaced by bonding with adhesive. The rotor 2 has a flank 21 at the upper part of its inner surface which is formed to prevent its contact with the yoke 19.
As shown in the partly enlarged sectional view in FIG. 2, the rotor 2, to which is fixed the polygon mirror 1, is inserted into the bearing 5. There is a clearance 27 between them which is of the order of several to ten-odd micrometers. It is in this clearance that the dynamic pressure occurs. The bearing 5 has a base 8 fixed thereto. The center column of the base 8 has a rotor driving coil 9 fitted thereto. This coil 9 and the annular magnet 10 constitute the motor drive. The center column of the base 8 has an annular magnet 11 attached to its outer surface at its upper part. The balancing plate 16 also has an annular magnet 12 attached to its inner surface. These two annular magnets 11 and 12 face each other, with their polarity opposite in the axial direction, so that they constitute a magnetic thrust bearing.
Certain problems exist with the above constructions. Since the rotor 2 is inserted into the bearing 5, with a clearance for dynamic pressure between, as mentioned above, the rotor 2 slides on the bearing 5 when the rotor starts before dynamic pressure occurs or when the rotor stops after dynamic pressure has disappeared. Sliding causes wear to the rotor and bearing. In addition, there is a possibility of the rotor 2 coming into contact with the bearing 5 during operation due to external turbulence. This would damage the dynamic-pressure surface.
In the conventional case where the dynamic-pressure pneumatic bearing is made of aluminum alloy, the problem associated with sliding (which causes wear and seizure) is approached by surface treatment such as electroless composite plating of SiC and nickel and lubricating anodized film. Such surface treatment provides good wear resistance but does not protect the dynamic-pressure surface from damage when the rotor comes into contact with the bearing during operation. Damage to the dynamic-pressure surface leads to seizure, which stops rotation, in the worst case. This is probable when the motor is mounted on a movable part.
One way of protecting the dynamic-pressure surface from damage is to make the bearing from quenched stainless steel or ceramics; however, such a material differs in thermal expansibility from aluminum alloy from which the polygon mirror is made. This creates an imbalance as the temperature changes.
Moreover, in the conventional case, the dynamic pressure pneumatic (fluid) bearing has poor jitter characteristic, and in the worst case, seizure results from jitter.
The volume resistivity of a coating layer in the dynamic pressure surfaces of bearing parts is high resulting in dust or powder to stick to the dynamic-pressure surfaces due to, for example, static electricity. While the motor rotates, the dust or powder makes contacts with the dynamic pressure surfaces and wears those surfaces. A conventional layer includes graphite, carbon black, which are a kind of conductive inorganic filler, but such conventional layer has 3.times.10.sup.12 .OMEGA. cm (volume resitivity).
The volume resistivity of the conventional layer is high and an electrification makes it easy. As a result, there is adhesion of dust, etc., due to static electricity. During assembly, the dust remains on the dynamic pressure surfaces of the bearing parts (e.g., the space between a shaft and a bearing) resulting in a deteriorated jitter characteristic of the motor. Although the parts are cleaned during assembly, e.g., by use of an air gun, such cleaning is insufficient to completely remove the dust.
When the motor rotates, the remaining dust remains on the parts due to high static electricity. If the dust includes a hard foreign matter, then that foreign matter wears the layer that formed the shaft or the bearing. In the worst case, the motor seizures.
The resin powder wears the parts when the dynamic pressure surfaces slide on one another. Spaces form from the wear and the the dust and foreign matter move into the spaces making it even more difficult to remove the dust and foreign matter.